Magnetic tape device and head tracking servo method

ABSTRACT

The magnetic tape device includes: a magnetic tape; and a servo head, in which a magnetic tape transportation speed of the magnetic tape device is equal to or lower than 18 m/sec, the servo head is a magnetic head including a tunnel magnetoresistance effect type element as a servo pattern reading element, the magnetic tape includes a non-magnetic support, and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, the magnetic layer includes a servo pattern, and a coefficient of friction measured regarding a base portion of a surface of the magnetic layer is equal to or smaller than 0.30.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2016-254439 filed on Dec. 27, 2016. The aboveapplication is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape device and a headtracking servo method.

2. Description of the Related Art

Magnetic recording is used as a method of recording information in arecording medium. In the magnetic recording, information is recorded ona magnetic recording medium as a magnetized pattern. Informationrecorded on a magnetic recording medium is reproduced by reading amagnetic signal obtained from the magnetized pattern by a magnetic head.As a magnetic head used for such reproducing, various magnetic headshave been proposed (for example, see JP2004-185676A).

SUMMARY OF THE INVENTION

An increase in recording capacity (high capacity) of a magneticrecording medium is required in accordance with a great increase ininformation content in recent years. As means for realizing highcapacity, a technology of increasing a recording density of a magneticrecording medium is used. However, as the recording density increases, amagnetic signal (specifically, a leakage magnetic field) obtained from amagnetic layer tends to become weak. Accordingly, it is desired that ahigh-sensitivity magnetic head capable of reading a weak signal withexcellent sensitivity is used as a reproducing head. Regarding thesensitivity of the magnetic head, it is said that a magnetoresistive(MR) head using a magnetoresistance effect as an operating principle hasexcellent sensitivity, compared to an inductive head used in the relatedart.

As the MR head, an anisotropic magnetoresistive (AMR) head and a giantmagnetoresistive (GMR) head are known as disclosed in a paragraph 0003of JP2004-185676A. The GMR head is an MR head having excellentsensitivity than that of the AMR head. In addition, a tunnelmagnetoresistive (TMR) head disclosed in a paragraph 0004 and the likeof JP2004-185676A is an MR head having a high possibility of realizinghigher sensitivity.

Meanwhile, a recording and reproducing system of the magnetic recordingis broadly divided into a levitation type and a sliding type. A magneticrecording medium in which information is recorded by the magneticrecording is broadly divided into a magnetic disk and a magnetic tape.Hereinafter, a drive including a magnetic disk as a magnetic recordingmedium is referred to as a “magnetic disk device” and a drive includinga magnetic tape as a magnetic recording medium is referred to as a“magnetic tape device”.

The magnetic disk device is generally called a hard disk drive (HDD) anda levitation type recording and reproducing system is used. In themagnetic disk device, a shape of a surface of a magnetic head sliderfacing a magnetic disk and a head suspension assembly that supports themagnetic head slider are designed so that a predetermined intervalbetween a magnetic disk and a magnetic head can be maintained with airflow at the time of rotation of the magnetic disk. In such a magneticdisk device, information is recorded and reproduced in a state where themagnetic disk and the magnetic head do not come into contact with eachother. The recording and reproducing system described above is thelevitation type. On the other hand, a sliding type recording andreproducing system is used in the magnetic tape device. In the magnetictape device, a surface of a magnetic layer of a magnetic tape and amagnetic head come into contact with each other and slide on each other,at the time of the recording and reproducing information.

JP2004-185676A proposes usage of the TMR head as a reproducing head forreproducing information in the magnetic disk device. On the other hand,the usage of the TMR head as a reproducing head in the magnetic tapedevice is currently still in a stage where the future usage thereof isexpected, and the usage thereof is not yet practically realized.

However, in the magnetic tape, information is normally recorded on adata band of the magnetic tape. Accordingly, data tracks are formed inthe data band. As means for realizing high capacity of the magnetictape, a technology of disposing the larger amount of data tracks in awidth direction of the magnetic tape by narrowing the width of the datatrack to increase recording density is used. However, in a case wherethe width of the data track is narrowed and the recording and/orreproduction of information is performed by transporting the magnetictape in the magnetic tape device, it is difficult that a magnetic headproperly follows the data tracks in accordance with the position changeof the magnetic tape, and errors may easily occur at the time ofrecording and/or reproduction. Thus, as means for preventing occurrenceof such errors, a method of forming a servo pattern in the magneticlayer and performing head tracking servo has been recently proposed andpractically used. In a magnetic servo type head tracking servo amonghead tracking servos, a servo pattern is formed in a magnetic layer of amagnetic tape, and this servo pattern is read by a servo head to performhead tracking servo. The head tracking servo is to control a position ofa magnetic head in the magnetic tape device. The head tracking servo ismore specifically performed as follows.

First, a servo head reads a servo pattern to be formed in a magneticlayer (that is, reproduces a servo signal). A position of a magnetichead in a magnetic tape device is controlled in accordance with a valueobtained by reading the servo pattern. Accordingly, in a case oftransporting the magnetic tape in the magnetic tape device for recordingand/or reproducing information, it is possible to increase an accuracyof the magnetic head following the data track, even in a case where theposition of the magnetic tape is changed. For example, even in a casewhere the position of the magnetic tape is changed in the widthdirection with respect to the magnetic head, in a case of recordingand/or reproducing information by transporting the magnetic tape in themagnetic tape device, it is possible to control the position of themagnetic head of the magnetic tape in the width direction in themagnetic tape device, by performing the head tracking servo. By doingso, it is possible to properly record information in the magnetic tapeand/or properly reproduce information recorded on the magnetic tape inthe magnetic tape device.

The servo pattern is formed by magnetizing a specific position of themagnetic layer. A plurality of regions including a servo pattern(referred to as “servo bands”) are generally present in the magnetictape capable of performing the head tracking servo along a longitudinaldirection. A region interposed between two servo bands is referred to asa data band. The recording of information is performed on the data bandand a plurality of data tracks are formed in each data band along thelongitudinal direction. In order to realize high capacity of themagnetic tape, it is preferable that the larger number of the data bandswhich are regions where information is recorded are present in themagnetic layer. As means for that, a technology of increasing apercentage of the data bands occupying the magnetic layer by narrowingthe width of the servo band which is not a region in which informationis recorded is considered. In regards to this point, the inventors haveconsidered that, since a read track width of the servo pattern becomesnarrow, in a case where the width of the servo band becomes narrow, itis desired to use a magnetic head having high sensitivity as the servohead, in order to ensure signal-to-noise-ratio (SNR) at the time ofreading the servo pattern. As a magnetic head for this, the inventorsfocused on a TMR head which has been proposed to be used as areproducing head in the magnetic disk device in JP2004-185676A. Asdescribed above, the usage of the TMR head in the magnetic tape deviceis still in a stage where the future use thereof as a reproducing headfor reproducing information is expected, and the usage of the TMR headas the servo head has not even proposed yet. However, the inventors havethought that, it is possible to deal with realization of highersensitivity of the future magnetic tape, in a case where the TMR head isused as the servo head in the magnetic tape device which performs thehead tracking servo.

That is, an object of one aspect of the invention is to provide amagnetic tape device in which a TMR head is mounted as a servo head.

A magnetoresistance effect which is an operating principle of the MRhead such as the TMR head is a phenomenon in which electric resistancechanges depending on a change in magnetic field. The MR head detects achange in leakage magnetic field generated from a magnetic recordingmedium as a change in resistance value (electric resistance) andreproduces information by converting the change in resistance value intoa change in voltage. In a case where the TMR head is used as the servohead, the TMR head detects a change in leakage magnetic field generatedfrom a magnetic layer in which the servo pattern is formed, as a changein resistance value (electric resistance) and reads the servo pattern(reproduces a servo signal) by converting the change in resistance valueinto a change in voltage. It is said that a resistance value of the TMRhead is generally high, as disclosed in a paragraph 0007 ofJP2004-185676A, but occurrence of a significant decrease in resistancevalue in the TMR head may cause a decrease in sensitivity of the TMRhead, thereby resulting in a decrease in signal intensity of a servosignal reproduced by the servo head and a decrease in SNR accompaniedwith that. Accordingly, the accuracy of the head position controlling ofthe head tracking servo may decrease.

During intensive studies for achieving the object described above, theinventors have found a phenomenon which was not known in the relatedart, in that, in a case of using the TMR head as a servo head in themagnetic tape device which performs the head tracking servo, asignificant decrease in resistance value (electric resistance) occurs inthe TMR head. A decrease in resistance value of the TMR head is adecrease in electric resistance measured by bringing an electricresistance measuring device into contact with a wiring connecting twoelectrodes configuring a tunnel magnetoresistance effect type elementincluded in the TMR head. The phenomenon in which this resistance valuesignificantly decreases is not observed in a case of using the TMR headin the magnetic disk device, nor in a case of using other MR heads suchas the GMR head in the magnetic disk device or the magnetic tape device.That is, occurrence of a significant decrease in resistance value in theTMR head in a case of using the TMR head was not even confirmed in therelated art. A difference in the recording and reproducing systembetween the magnetic disk device and the magnetic tape device,specifically, contact and non-contact between a magnetic recordingmedium and a magnetic head may be the reason why a significant decreasein resistance value of the TMR head occurred in the magnetic tape deviceis not observed in the magnetic disk device. In addition, the TMR headhas a special structure in which two electrodes are provided with aninsulating layer (tunnel barrier layer) interposed therebetween in adirection in which a magnetic tape is transported, which is not appliedto other MR heads which are currently practically used, and it isconsidered that this is the reason why a significant decrease inresistance value occurring in the TMR head is not observed in other MRheads.

With respect to this, as a result of more intensive studies afterfinding the phenomenon described above, the inventors have newly foundthe following points.

It is desired that the magnetic tape is transported at a low speed inthe magnetic tape device, in order to prevent a deterioration inrecording and reproducing characteristics. But, in a case where themagnetic tape is transported at a low speed which is equal to or lowerthan a predetermined speed in the magnetic tape device (specifically, ina case where a magnetic tape transportation speed is equal to or lowerthan 18 m/sec), a decrease in resistance value of the TMR head whichreads a servo pattern for performing head tracking servo at the time ofrecording and/or reproduction of information particularly significantlyoccurs.

However, such a decrease in resistance value can be prevented by using amagnetic tape described below as the magnetic tape.

One aspect of the invention has been completed based on the findingdescribed above.

That is, according to one aspect of the invention, there is provided amagnetic tape device comprising: a magnetic tape; and a servo head, inwhich a magnetic tape transportation speed of the magnetic tape deviceis equal to or lower than 18 m/sec, the servo head is a magnetic head(hereinafter, also referred to as a “TMR head”) including a tunnelmagnetoresistance effect type element (hereinafter, also referred to asa “TMR element”) as a servo pattern reading element, the magnetic tapeincludes a non-magnetic support, and a magnetic layer includingferromagnetic powder and a binding agent on the non-magnetic support,the magnetic layer includes a servo pattern, and a coefficient offriction measured regarding a base portion of a surface of the magneticlayer, is equal to or smaller than 0.30.

According to another aspect of the invention, there is provided a headtracking servo method comprising: reading a servo pattern of a magneticlayer of a magnetic tape by a servo head in a magnetic tape device, inwhich a magnetic tape transportation speed of the magnetic tape deviceis equal to or lower than 18 m/sec, the servo head is a magnetic headincluding a tunnel magnetoresistance effect type element as a servopattern reading element, the magnetic tape includes a non-magneticsupport, and a magnetic layer including ferromagnetic powder and abinding agent on the non-magnetic support, the magnetic layer includesthe servo pattern, and a coefficient of friction measured regarding abase portion of a surface of the magnetic layer, is equal to or smallerthan 0.30.

The “base portion” of the invention and the specification is a portionof the surface of the magnetic layer of the magnetic tape specified bythe following method. In the invention and the specification, the“surface of the magnetic layer” of the magnetic tape is identical to thesurface of the magnetic tape on the magnetic layer side. A surface onwhich volume of a protrusion and volume of a recess in a visual fieldmeasured by an atomic force microscope (AFM) are identical to each otheris determined as a reference surface. A projection having a height equalto or greater than 15 nm from the reference surface is defined as aprojection. A portion in which the number of such projections is zero,that is, a portion of the surface of the magnetic layer of the magnetictape in which a projection having a height equal to or greater than 15nm from the reference surface is not detected is specified as the baseportion.

A coefficient of friction measured regarding the base portion is a valuemeasured by the following method.

In the base portion (measured part: length of the magnetic tape in alongitudinal direction of 10 μm), a diamond spherical indenter having aradius of 1 μm is allowed to reciprocate once with a load of 100 μN anda speed of 1 μm/sec to measure a frictional force (horizontal force) anda normal force. The frictional force and the normal force measured hereare an arithmetical mean of respective values obtained by continuouslymeasuring frictional forces and normal forces during the onereciprocating operation. The measurement described above can beperformed with TI-950 type TRIBOINDENTER manufactured by Hysitron, Inc.A value of a coefficient of friction μ is calculated from anarithmetical mean of the frictional forces and an arithmetical mean ofthe normal forces measured as described above. The coefficient offriction is a value measured by an equation of F=μN, from the frictionalforce (horizontal force) F (unit: newton (N)) and the normal force N(unit: newton (N)). The measurement and the calculation of the value ofthe coefficient of friction μ are performed at three portions of thebase portion arbitrarily selected from the surface of the magnetic layerof the magnetic tape, and an arithmetical mean of the three measuredvalues obtained is set as a coefficient of friction measured regardingthe base portion. Hereinafter, the coefficient of friction measuredregarding the base portion is also referred to as a “base friction”.

One aspect of the magnetic tape device and the head tracking servomethod is as follows.

In one aspect, the coefficient of friction measured regarding the baseportion of the surface of the magnetic layer is 0.20 to 0.30.

In one aspect, a center line average surface roughness Ra measuredregarding a surface of the magnetic layer is equal to or smaller than2.8 nm.

In the invention and the specification, the center line average surfaceroughness Ra measured regarding the surface of the magnetic layer of themagnetic tape is a value measured with an atomic force microscope in aregion having an area of 40 μm×40 μm. As an example of the measurementconditions, the following measurement conditions can be used. The centerline average surface roughness Ra shown in examples which will bedescribed later is a value obtained by the measurement under thefollowing measurement conditions.

The measurement is performed regarding the region of 40 μm×40 μm of thearea of the surface of the magnetic layer of the magnetic tape with anAFM (Nanoscope 4 manufactured by Veeco Instruments, Inc.). A scan speed(probe movement speed) is set as 40 μm/sec and a resolution is set as512 pixel×512 pixel.

In one aspect, the center line average surface roughness Ra is equal toor smaller than 2.5 nm.

In one aspect, the magnetic tape includes a non-magnetic layer includingnon-magnetic powder and a binding agent between the non-magnetic supportand the magnetic layer, and a total thickness of the magnetic layer andthe non-magnetic layer is equal to or smaller than 1.8 μm.

In one aspect, the total thickness of the magnetic layer and thenon-magnetic layer is equal to or smaller than 1.1 μm.

According to one aspect of the invention, it is possible to preventoccurrence of a significant decrease in resistance value in the TMRhead, in a case of reading a servo pattern of the magnetic layer of themagnetic tape which is transported at the magnetic tape transportationspeed equal to or lower than 18 m/sec, by the TMR head.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of disposition of data bands and servo bands.

FIG. 2 shows a servo pattern disposition example of a linear-tape-open(LTO) Ultrium format tape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Tape Device

One aspect of the invention relates to a magnetic tape device including:a magnetic tape; and a servo head, in which a magnetic tapetransportation speed of the magnetic tape device is equal to or lowerthan 18 m/sec, the servo head is a magnetic head including a tunnelmagnetoresistance effect type element as a servo pattern readingelement, the magnetic tape includes a non-magnetic support, and amagnetic layer including ferromagnetic powder and a binding agent on thenon-magnetic support, the magnetic layer includes a servo pattern, and acoefficient of friction measured regarding a base portion of a surfaceof the magnetic layer, is equal to or smaller than 0.30.

The magnetic tape transportation speed of the magnetic tape device isalso referred to as a running speed. In the invention and thespecification, the “magnetic tape transportation speed” is a relativespeed between the magnetic tape transported in the magnetic tape deviceand the servo head in a case where the servo pattern is read by theservo head. As a result of intensive studies of the inventors, theinventors have newly found that it is possible to prevent a phenomenonin which a decrease in resistance value of the TMR head used as theservo head in the magnetic tape device in which the magnetic tapetransportation speed is equal to or lower than 18 m/sec, occursparticularly significantly, by using the magnetic tape which includesthe magnetic layer including ferromagnetic powder and a binding agent onthe non-magnetic support, and in which the coefficient of frictionmeasured regarding the base portion of the surface of the magnetic layeris equal to or smaller than 0.30. This point will be further describedbelow.

In the magnetic tape device, in a case of using a magnetic tape of therelated art, in a case where the TMR head is used as the servo headunder specific conditions in which the magnetic tape transportationspeed is equal to or lower than 18 m/sec, a phenomenon in which aresistance value (electric resistance) significantly decreases occurs inthe TMR head used as the servo head. This phenomenon is a phenomenonthat is newly found by the inventors. The inventors have considered thereason for the occurrence of such a phenomenon is as follows.

The TMR head is a magnetic head using a tunnel magnetoresistance effectand includes two electrodes with an insulating layer (tunnel barrierlayer) interposed therebetween. The tunnel barrier layer positionedbetween the two electrodes is an insulating layer, and thus, even in acase where a voltage is applied between the two electrodes, in general,a current does not flow or does not substantially flow between theelectrodes. However, a current (tunnel current) flows by a tunnel effectdepending on a direction of a magnetic field of a free layer affected bya leakage magnetic field from the magnetic tape, and a change in amountof a tunnel current flow is detected as a change in resistance value bythe tunnel magnetoresistance effect. By converting the change inresistance value into a change in voltage, a servo pattern formed in themagnetic tape can be read (a servo signal can be reproduced).

Examples of a structure of the MR head include a current-in-plane (CIP)structure and a current-perpendicular-to-plane (CPP) structure, and theTMR head is a magnetic head having a CPP structure. In the MR headhaving a CPP structure, a current flows in a direction perpendicular toa film surface of an MR element, that is, a direction in which themagnetic tape is transported, in a case of reading a servo patternformed in the magnetic tape. With respect to this, other MR heads, forexample, a spin valve type GMR head which is widely used in recent yearsamong the GMR heads has a CIP structure. In the MR head having a CIPstructure, a current flows in a direction in a film plane of an MRelement, that is, a direction perpendicular to a direction in which themagnetic tape is transported, in a case of reading a servo patternformed in the magnetic tape.

As described above, the TMR head has a special structure which is notapplied to other MR heads which are currently practically used.Accordingly, in a case where short circuit (bypass due to damage) occurseven at one portion between the two electrodes, the resistance valuesignificantly decreases. A significant decrease in resistance value in acase of the short circuit occurred even at one portion between the twoelectrodes as described above is a phenomenon which does not occur inother MR heads. In the magnetic disk device using a levitation typerecording and reproducing system, a magnetic disk and a magnetic head donot come into contact with each other, and thus, damage causing shortcircuit hardly occurs. On the other hand, in the magnetic tape deviceusing a sliding type recording and reproducing system, the magnetic tapeand the servo head come into contact with each other and slide on eachother, in a case of reading a servo pattern by the servo head.Accordingly, the TMR head is damaged due to the sliding between the TMRhead and the magnetic tape, and thus, short circuit easily occurs. Amongthese, the inventors have thought that, in a case where thetransportation speed of the magnetic tape is as low as a speed equal toor lower than 18 m/sec, a possibility that the TMR head comes intocontact with the base portion of the surface of the magnetic layer in acase of reading a servo pattern increases, compared to a case where thetransportation speed of the magnetic tape exceeds 18 m/sec. Accordingly,the inventors have surmised that, in a case where any measures are notprepared, the TMR head is affected by the contact with the base portionand the TMR head is easily damaged. The inventors have assumed that thisis the reason why a decrease in resistance value of the TMR head occursparticularly significantly in a case of using the TMR head as the servohead in the magnetic tape device in which the magnetic tapetransportation speed is equal to or lower than 18 m/sec.

With respect to this, according to the magnetic tape in which acoefficient of friction (base friction) measured regarding the baseportion of the surface of the magnetic layer is equal to or smaller than0.30, it is possible to allow the TMR head coming into contact with thebase portion to smoothly slide on the base portion. Accordingly, theinventors have surmised that the reducing of the effect on the TMR headdue to the contact with the base portion contributes to the preventionof occurrence of short circuit due to a damage on the TMR head.

However, the above descriptions are merely a surmise of the inventorsand the invention is not limited thereto.

Regarding the base friction, JP2016-071912A discloses that the basefriction is set to be in a specific range, in order to prevent adecrease in electromagnetic conversion characteristics of a thinnedmagnetic tape during repeated running. However, as described above, theusage of the TMR head as a servo head in the magnetic tape device is noteven proposed in the related art. In addition, in the magnetic tapedevice in which the TMR head is mounted as a servo head, the occurrenceof a particularly significant decrease in resistance value of the TMRhead at a specific magnetic tape transportation speed (specifically,equal to or lower than 18 m/sec) is a phenomenon which was not known inthe related art. With respect to such a phenomenon, the effect of thebase friction and a possibility of prevention of the phenomenon bysetting the base friction to be equal to or smaller than 0.30 are notdisclosed in JP2016-071912A and is newly found by the inventors as aresult of intensive studies.

Hereinafter, the magnetic tape device will be described morespecifically. A “decrease in resistance value of the TMR head” describedbelow is a significant decrease in resistance value of the TMR headoccurring in a case of reading a servo pattern by using the TMR head asthe servo head, in the magnetic tape device having the magnetic tapetransportation speed equal to or lower than 18 m/sec, unless otherwisenoted.

Magnetic Tape

Base Friction

The coefficient of friction (base friction) measured regarding the baseportion of the surface of the magnetic layer of the magnetic tape isequal to or smaller than 0.30, from a viewpoint of preventing a decreasein resistance value of the TMR head, and is preferably equal to orsmaller than 0.28 and more preferably equal to or smaller than 0.26,from a viewpoint of further preventing a decrease in resistance value ofthe TMR head. The base friction can be, for example, equal to or greaterthan 0.10, equal to or greater than 0.15, or equal to or greater than0.20. However, from a viewpoint of preventing a decrease in resistancevalue of the TMR head, a low base friction is preferable, and thus, thebase friction may be smaller than the values described above.

In the measurement method of the base friction described above, thereason why a projection having a height equal to or greater than 15 nmfrom the reference surface is defined as a projection is because,normally, a projection recognized as a projection present on the surfaceof the magnetic layer is mainly a projection having a height equal to orgreater than 15 nm from the reference surface. Such a projection is, forexample, formed of non-magnetic powder such as an abrasive on thesurface of the magnetic layer. With respect to this, it is consideredthat more microscopic ruggedness than ruggedness formed by such aprojection is present on the surface of the magnetic layer. Theinventors have surmised that it is possible to adjust the base frictionby controlling a shape of the microscopic ruggedness. As a method forrealizing the adjustment described above, a method of using two or morekinds of ferromagnetic powders having different average particle sizesas the ferromagnetic powder is used. More specifically, it is thoughtthat, the microscopic ruggedness can be formed on the base portion, in acase where the ferromagnetic powder having a greater average particlesize becomes a protrusion, and it is possible to increase a percentageof the protrusion present on the base portion by increasing a mixingpercentage of the ferromagnetic powder having a greater average particlesize (or, conversely, to decrease a percentage of protrusion present onthe base portion by decreasing the mixing percentage). Such a methodwill be described later more specifically.

As another method, a method of forming a magnetic layer with othernon-magnetic powder having a greater average particle size than that offerromagnetic powder, in addition to non-magnetic powder such as anabrasive which can form a projection having a height equal to or greaterthan 15 nm from the reference surface on the surface of the magneticlayer. More specifically, it is thought that, the microscopic ruggednesscan be formed on the base portion, in a case where the othernon-magnetic powder becomes a protrusion, and it is possible to increasea percentage of the protrusion present on the base portion by increasinga mixing percentage of the non-magnetic powder (or, conversely, todecrease a percentage of protrusion present on the base portion bydecreasing the mixing percentage). Such a method will be described latermore specifically.

In addition, it is also possible to adjust the base friction bycombining the two kinds of methods.

However, the adjustment methods are merely examples, and it is possibleto realize a base friction equal to or smaller than 0.30 by an arbitrarymethod capable of adjusting the base friction, and such an aspect isalso included in the invention.

Next, the magnetic layer and the like included in the magnetic tape willbe described more specifically.

Magnetic Layer

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer,ferromagnetic powder normally used in the magnetic layer of variousmagnetic recording media can be used.

As described above, as a method of adjusting the base friction, a methodof forming a magnetic layer with two or more kinds of ferromagneticpowders having different average particle sizes as ferromagnetic powderis used. In this case, it is preferable that the ferromagnetic powderhaving a smaller average particle size is used as ferromagnetic powderused with the largest proportion, among the two or more kinds offerromagnetic powder, from a viewpoint of improving recording density ofthe magnetic tape. From this viewpoint, in a case where two or morekinds of ferromagnetic powders having different average particle sizesare used as ferromagnetic powder of a magnetic layer, it is preferablethat the ferromagnetic powder having an average particle size equal toor smaller than 50 nm is used as the ferromagnetic powder used with thelargest proportion. On the other hand, it is preferable that an averageparticle size of the ferromagnetic powder used with the largestproportion is equal to or greater than 10 nm, from a viewpoint ofstability of magnetization. In a case of using one of ferromagneticpowder without using two or more kinds of ferromagnetic powders havingdifferent average particle sizes, the average particle size of theferromagnetic powder is preferably equal to or smaller than 50 nm andmore preferably equal to or greater than 10 nm, due to the reasonsdescribed above.

With respect to this, it is preferable that the ferromagnetic powderused with the ferromagnetic powder used with the largest proportion hasa greater average particle size than that of the ferromagnetic powderused with the largest proportion. This may be because the base frictioncan be decreased due to the protrusion formed of the ferromagneticpowder having a great average particle size on the base portion. Fromthis viewpoint, a difference between an average particle size of theferromagnetic powder used with the largest proportion and an averageparticle size of the ferromagnetic powder used therewith, acquired as“(latter average particle size)−(former average particle size)” ispreferably 10 to 80 nm, more preferably 10 to 50 nm, even morepreferably 10 to 40 nm, and still more preferably 12 to 35 nm. As theferromagnetic powder used with the ferromagnetic powder used with thelargest proportion, it is also possible to use two or more kinds offerromagnetic powders having different average particle sizes. In thiscase, it is preferable that an average particle size of at least one offerromagnetic powder of the two or more kinds of ferromagnetic powderssatisfies the difference described above, it is more preferable thataverage particle sizes of more kinds of ferromagnetic powders satisfythe difference described above, and it is even more preferable thataverage particle sizes of all of the ferromagnetic powders satisfy thedifference described above, with respect to the average particle size ofthe ferromagnetic powder used with the largest proportion.

Regarding two or more kinds of ferromagnetic powders having differentaverage particle sizes, from a viewpoint of controlling base friction, amixing ratio of the ferromagnetic powder used with the largestproportion to the other ferromagnetic powder (in a case of using two ormore kinds of ferromagnetic powders having different average particlesizes as other ferromagnetic powder, the total thereof), is preferably90.0:10.0 (former:latter) to 99.9:0.1 and more preferably 95.0:5.0 to99.5:0.5 based on mass.

Here, the ferromagnetic powders having different average particle sizesindicate the total or a part of a batch of the ferromagnetic powdershaving different average particle sizes. In a case where particle sizedistribution based on the number or volume of the ferromagnetic powderincluded in the magnetic layer of the magnetic tape formed with theferromagnetic powders having different average particle sizes asdescribed above is measured by a well-known measurement method such as adynamic light scattering method or a laser diffraction method, anaverage particle size or a maximum peak in the vicinity thereof of theferromagnetic powder used with the largest proportion can be normallyconfirmed in a particle size distribution curve obtained by themeasurement. In addition, an average particle size or a peak in thevicinity thereof of each ferromagnetic powder may be confirmed.Accordingly, in a case where the particle size distribution of theferromagnetic powder included in the magnetic layer of the magnetic tapeformed by using ferromagnetic powder having an average particle size of10 to 50 nm with the largest proportion, for example, is measured, themaximum peak can be generally confirmed in a range of the particle sizeof 10 to 50 nm in the particle size distribution curve.

A part of the other ferromagnetic powders described above may besubstituted with other non-magnetic powder which will be describedlater.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic powder and the like are valuesmeasured by the following method with a transmission electronmicroscope, unless otherwise noted.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, the image is printed on printing paperso that the total magnification of 500,000 to obtain an image ofparticles configuring the powder. A target particle is selected from theobtained image of particles, an outline of the particle is traced with adigitizer, and a size of the particle (primary particle) is measured.The primary particle is an independent particle which is not aggregated.

The measurement described above is performed regarding 500 particlesarbitrarily extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss.

In the invention and the specification, the average particle size of theferromagnetic powder and other powder is an average particle sizeobtained by the method described above, unless otherwise noted. Theaverage particle size shown in examples which will be described later isa value measured by using transmission electron microscope H-9000manufactured by Hitachi, Ltd. as the transmission electron microscope,and image analysis software KS-400 manufactured by Carl Zeiss as theimage analysis software, unless otherwise noted. In the invention andthe specification, the powder means an aggregate of a plurality ofparticles. For example, the ferromagnetic powder means an aggregate of aplurality of ferromagnetic particles. The aggregate of the plurality ofparticles not only includes an aspect in which particles configuring theaggregate directly come into contact with each other, and also includesan aspect in which a binding agent or an additive which will bedescribed later is interposed between the particles. A term “particles”is also used for describing the powder.

As a method of collecting a sample powder from the magnetic tape inorder to measure the particle size, a method disclosed in a paragraph of0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in acase where the shape of the particle observed in the particle imagedescribed above is a needle shape, a fusiform shape, or a columnar shape(here, a height is greater than a maximum long diameter of a bottomsurface), the size (particle size) of the particles configuring thepowder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a planar shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, in a case of the definition (2), the average particle size is anaverage plate diameter, and an average plate ratio is an arithmeticalmean of (maximum long diameter/thickness or height). In a case of thedefinition (3), the average particle size is an average diameter (alsoreferred to as an average particle diameter).

As a preferred specific example of the ferromagnetic powder,ferromagnetic hexagonal ferrite powder can be used. In a case where theferromagnetic powder used with the largest proportion is ferromagnetichexagonal ferrite powder, an average particle size thereof is preferably10 nm to 50 nm and more preferably 20 nm to 50 nm, from a viewpoint ofrealization of high-density recording and stability of magnetization.For details of the ferromagnetic hexagonal ferrite powder, descriptionsdisclosed in paragraphs 0012 to 0030 of JP2011-225417A, paragraphs 0134to 0136 of JP2011-216149A, and paragraphs 0013 to 0030 of JP2012-204726Acan be referred to, for example.

As a preferred specific example of the ferromagnetic powder,ferromagnetic metal powder can also be used. In a case where theferromagnetic powder used with the largest proportion is ferromagneticmetal powder, an average particle size thereof is preferably 10 nm to 50nm and more preferably 20 nm to 50 nm, from a viewpoint of realizationof high-density recording and stability of magnetization. For details ofthe ferromagnetic metal powder, descriptions disclosed in paragraphs0137 to 0141 of JP2011-216149A and paragraphs 0009 to 0023 ofJP2005-251351A can be referred to, for example.

As the ferromagnetic powder, the magnetic tape may include only one offerromagnetic hexagonal ferrite powder and ferromagnetic metal powder,may include both thereof, or may include other ferromagnetic powderswith one or both thereof.

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably 50 to 90 mass % and more preferably 60 to90 mass %. The components other than the ferromagnetic powder of themagnetic layer are at least a binding agent and one or more kinds ofadditives may be arbitrarily included. A high filling percentage of theferromagnetic powder in the magnetic layer is preferable from aviewpoint of improvement recording density.

Binding Agent

The magnetic tape is a coating type magnetic tape, and the magneticlayer includes a binding agent together with the ferromagnetic powder.As the binding agent, one or more kinds of resin is used. The resin maybe a homopolymer or a copolymer. As the binding agent, various resinsnormally used as a binding agent of the coating type magnetic recordingmedium can be used. For example, as the binding agent, a resin selectedfrom a polyurethane resin, a polyester resin, a polyamide resin, a vinylchloride resin, an acrylic resin obtained by copolymerizing styrene,acrylonitrile, or methyl methacrylate, a cellulose resin such asnitrocellulose, an epoxy resin, a phenoxy resin, and a polyvinylalkylalresin such as polyvinyl acetal or polyvinyl butyral can be used alone ora plurality of resins can be mixed with each other to be used. Amongthese, a polyurethane resin, an acrylic resin, a cellulose resin, and avinyl chloride resin are preferable. These resins can be used as thebinding agent even in the non-magnetic layer and/or a back coating layerwhich will be described later. For the binding agent described above,description disclosed in paragraphs 0028 to 0031 of JP2010-24113A can bereferred to. An average molecular weight of the resin used as thebinding agent can be, for example, 10,000 to 200,000 as a weight-averagemolecular weight. The weight-average molecular weight of the inventionand the specification is a value obtained by performing polystyreneconversion of a value measured by gel permeation chromatography (GPC).As the measurement conditions, the following conditions can be used. Theweight-average molecular weight shown in examples which will bedescribed later is a value obtained by performing polystyrene conversionof a value measured under the following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmlD (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In addition, a curing agent can also be used together with the bindingagent. As the curing agent, in one aspect, a thermosetting compoundwhich is a compound in which a curing reaction (crosslinking reaction)proceeds due to heating can be used, and in another aspect, aphotocurable compound in which a curing reaction (crosslinking reaction)proceeds due to light irradiation can be used. At least a part of thecuring agent is included in the magnetic layer in a state of beingreacted (crosslinked) with other components such as the binding agent,by proceeding the curing reaction in the magnetic layer forming step.The preferred curing agent is a thermosetting compound, polyisocyanateis suitable. For details of the polyisocyanate, descriptions disclosedin paragraphs 0124 and 0125 of JP2011-216149A can be referred to, forexample. The amount of the curing agent can be, for example, 0 to 80.0parts by mass with respect to 100.0 parts by mass of the binding agentin the magnetic layer forming composition, and is preferably 50.0 to80.0 parts by mass, from a viewpoint of improvement of strength of eachlayer such as the magnetic layer.

Other Components

The magnetic layer includes ferromagnetic powder and a binding agent andif necessary, may include one or more kinds of additives. As theadditives, the curing agent described above is used as an example. Inaddition, examples of the additive which can be included in the magneticlayer include non-magnetic powder, a lubricant, a dispersing agent, adispersing assistant, an antifungal agent, an antistatic agent, anantioxidant, and carbon black. As the additives, a commerciallyavailable product can be suitably selected and used according to thedesired properties.

The magnetic layer preferably includes one or two or more kinds ofnon-magnetic powders. As the non-magnetic powder, non-magnetic powder(hereinafter, referred to as a “projection formation agent”) which canfunction as a projection formation agent which forms projectionssuitably protruding from the surface of the magnetic layer can be used.The projection formation agent is a component which can contribute tothe control of friction properties of the surface of the magnetic layerof the magnetic tape. The magnetic layer may include non-magnetic powder(hereinafter, referred to as an “abrasive”) which can function as anabrasive can be used. The magnetic layer of the magnetic tape preferablyincludes at least one of the projection formation agent or the abrasiveand more preferably includes both thereof.

Non-Magnetic Filler

As the projection formation agent which is one aspect of thenon-magnetic filler, various non-magnetic powders normally used as aprojection formation agent can be used. These may be inorganicsubstances or organic substances. In one aspect, from a viewpoint ofhomogenization of friction properties, particle size distribution of theprojection formation agent is not polydispersion having a plurality ofpeaks in the distribution and is preferably monodisperse showing asingle peak. From a viewpoint of availability of monodisperse particles,the projection formation agent is preferably powder of inorganicsubstances (inorganic powder). Examples of the inorganic powder includepowder of metal oxide, metal carbonate, metal sulfate, metal nitride,metal carbide, and metal sulfide, and powder of inorganic oxide ispreferable. The projection formation agent is more preferably colloidalparticles and even more preferably inorganic oxide colloidal particles.In addition, from a viewpoint of availability of monodisperse particles,the inorganic oxide configuring the inorganic oxide colloidal particlesare preferably silicon dioxide (silica). The inorganic oxide colloidalparticles are more preferably colloidal silica (silica colloidalparticles). In the invention and the specification, the “colloidalparticles” are particles which are not precipitated and dispersed togenerate a colloidal dispersion, in a case where 1 g of the particles isadded to 100 mL of at least one organic solvent of at least methyl ethylketone, cyclohexanone, toluene, or ethyl acetate, or a mixed solventincluding two or more kinds of the solvent described above at anarbitrary mixing ratio. The average particle size of the colloidalparticles is a value obtained by a method disclosed in a paragraph 0015of JP2011-048878A as a measurement method of an average particlediameter. In addition, in another aspect, the projection formation agentis preferably carbon black.

An average particle size of the projection formation agent is, forexample, 30 to 300 nm and is preferably 40 to 200 nm.

Meanwhile, the abrasive is preferably non-magnetic powder having Mohshardness exceeding 8 and more preferably non-magnetic powder having Mohshardness equal to or greater than 9. A maximum value of Mohs hardness is10 of diamond. Specifically, powders of alumina (Al₂O₃), siliconcarbide, boron carbide (BC), SiO₂, TiC, chromium oxide (Cr₂O₃), ceriumoxide, zirconium oxide (ZrO₂), iron oxide, diamond, and the like can beused, and among these, alumina powder such as α-alumina and siliconcarbide powder are preferable. In addition, regarding the particle sizeof the abrasive, a specific surface area which is an index for theparticle size is, for example, equal to or greater than 14 m²/g, and ispreferably 16 m²/g and more preferably 18 m²/g. Further, the specificsurface area of the abrasive can be, for example, equal to or smallerthan 40 m²/g. The specific surface area is a value obtained by anitrogen adsorption method (also referred to as a Brunauer-Emmett-Teller(BET) 1 point method), and is a value measured regarding primaryparticles. Hereinafter, the specific surface area obtained by such amethod is also referred to as a BET specific surface area.

In addition, from a viewpoint that the projection formation agent andthe abrasive can exhibit the functions thereof in more excellent manner,the content of the projection formation agent of the magnetic layer ispreferably 1.0 to 4.0 parts by mass and more preferably 1.5 to 3.5 partsby mass with respect to 100.0 parts by mass of the ferromagnetic powder.Meanwhile, the content of the magnetic layer is preferably 1.0 to 20.0parts by mass, more preferably 3.0 to 15.0 parts by mass, and even morepreferably 4.0 to 10.0 parts by mass with respect to 100.0 parts by massof the ferromagnetic powder.

As an example of the additive which can be used in the magnetic layerincluding the abrasive, a dispersing agent disclosed in paragraphs 0012to 0022 of JP2013-131285A can be used as a dispersing agent forimproving dispersibility of the abrasive of the magnetic layer formingcomposition. It is preferable to improve dispersibility of the magneticlayer forming composition of the non-magnetic powder such as anabrasive, in order to decrease the center line average surface roughnessRa measured regarding the surface of the magnetic layer.

As described above, in order to control the base friction to be equal toor smaller than 0.30, other non-magnetic powders can also be used inaddition to the non-magnetic powder described above. Such non-magneticpowder preferably has Mohs hardness equal to or smaller than 8, andvarious non-magnetic powders normally used in the non-magnetic layer canbe used. Specifically, the non-magnetic layer is as described later. Asmore preferred non-magnetic powder, red oxide can be used. The Mohshardness of red oxide is approximately 6.

It is preferable that the other non-magnetic powder described above hasan average particle size greater than that of the ferromagnetic powder,in the same manner as the ferromagnetic powder used with theferromagnetic powder used with the largest proportion described above.This is because the base friction may decrease due to the protrusionformed of the other non-magnetic powder on the base portion. From aviewpoint, difference between an average particle size of theferromagnetic powder and an average particle size of the othernon-magnetic powder used therewith, acquired as “(latter averageparticle size)−(former average particle size)” is preferably 10 to 80 nmand more preferably 10 to 50 nm. In a case of using two or more kinds offerromagnetic powders having different average particle sizes as theferromagnetic powder, the ferromagnetic powder used for calculating adifference the average particle size thereof and the average particlesize of the other non-magnetic powder is ferromagnetic powder used withthe largest proportion among two or more kinds of ferromagnetic powders.As the other non-magnetic powder, it is also possible to use two or morekinds of non-magnetic powders having different average particle sizes.In this case, it is preferable that an average particle size of at leastone of non-magnetic powder of the two or more of non-magnetic powderssatisfies the difference described above, it is more preferable thataverage particle sizes of more kinds of non-magnetic powders satisfy thedifference described above, and it is even more preferable that averageparticle sizes of all of the non-magnetic powders satisfy the differencedescribed above, with respect to the average particle size of theferromagnetic powder.

From a viewpoint of controlling base friction, a mixing ratio of theferromagnetic powder to the other non-magnetic powder (in a case ofusing two or more kinds of non-magnetic powders having different averageparticle sizes as other non-magnetic powder, the total thereof), ispreferably 90.0:10.0 (former:latter) to 99.9:0.1 and more preferably95.0:5.0 to 99.5:0.5 based on mass.

Center Line Average Surface Roughness Ra Measured Regarding Surface ofMagnetic Layer

Increasing a surface smoothness of the magnetic layer in the magnetictape causes improvement of electromagnetic conversion characteristics.Regarding the surface smoothness of the magnetic layer, the center lineaverage surface roughness Ra measured regarding the surface of themagnetic layer can be an index.

In one aspect, the center line average surface roughness Ra measuredregarding the surface of the magnetic layer of the magnetic tape ispreferably equal to or smaller than 2.8 nm, more preferably equal to orsmaller than 2.5 nm, even more preferably equal to or smaller than 2.3nm, and still more preferably equal to or smaller than 2.0 nm, from aviewpoint of improving electromagnetic conversion characteristics.However, according to the studies of the inventors, it is found that, ina case where the center line average surface roughness Ra measuredregarding the surface of the magnetic layer is equal to or smaller than2.5 nm and any measures are not prepared, a decrease in resistance valueof the TMR head tends to occur even more significantly. However, even asignificant decrease in resistance value of the TMR head occurring in acase where the Ra is equal to or smaller than 2.5 nm can be prevented,in a case of the magnetic tape device according to one aspect of theinvention. In addition, the center line average surface roughness Rameasured regarding the surface of the magnetic layer can be equal to orgreater than 1.2 nm or equal to or greater than 1.3 nm. From a viewpointof improving electromagnetic conversion characteristics, a low value ofthe Ra is preferable, and thus, the Ra may be lower than the valuesdescribed above.

The surface smoothness of the magnetic layer, that is, the center lineaverage surface roughness Ra measured regarding the surface of themagnetic layer can be controlled by a well-known method. For example,the surface smoothness of the magnetic layer can be controlled byadjusting a size of various powder (for example, ferromagnetic powder,non-magnetic powder, and the like) included in the magnetic layer ormanufacturing conditions of the magnetic tape.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic tape mayinclude a magnetic layer directly on a non-magnetic support, or mayinclude a non-magnetic layer including non-magnetic powder and a bindingagent between the non-magnetic support and the magnetic layer. Thenon-magnetic powder used in the non-magnetic layer may be powder ofinorganic substances or powder of organic substances. In addition,carbon black and the like can be used. Examples of the inorganicsubstances include metal, metal oxide, metal carbonate, metal sulfate,metal nitride, metal carbide, and metal sulfide. These non-magneticpowder can be purchased as a commercially available product or can bemanufactured by a well-known method. For details thereof, descriptionsdisclosed in paragraphs 0146 to 0150 of JP2011-216149A can be referredto. For carbon black which can be used in the non-magnetic layer,descriptions disclosed in paragraphs 0040 and 0041 of JP2010-24113A canbe referred to. The content (filling percentage) of the non-magneticpowder of the non-magnetic layer is preferably 50 to 90 mass % and morepreferably 60 to 90 mass %.

In regards to other details of a binding agent or additives of thenon-magnetic layer, the well-known technology regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technology regarding the magneticlayer can be applied.

The non-magnetic layer of the magnetic tape also includes asubstantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m (100Oe), or a layer having a residual magnetic flux density equal to orsmaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100 Oe). It is preferable that the non-magnetic layer does not have aresidual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support will be described. As the non-magneticsupport (hereinafter, also simply referred to as a “support”),well-known components such as polyethylene terephthalate, polyethylenenaphthalate, polyamide, polyamide imide, aromatic polyamide subjected tobiaxial stretching are used. Among these, polyethylene terephthalate,polyethylene naphthalate, and polyamide are preferable. Coronadischarge, plasma treatment, easy-bonding treatment, or heatingtreatment may be performed with respect to these supports in advance.

Back Coating Layer

The magnetic tape can also include a back coating layer includingnon-magnetic powder and a binding agent on a surface side of thenon-magnetic support opposite to the surface provided with the magneticlayer. The back coating layer preferably includes any one or both ofcarbon black and inorganic powder. In regards to the binding agentincluded in the back coating layer and various additives which can bearbitrarily included in the back coating layer, a well-known technologyregarding the treatment of the magnetic layer and/or the non-magneticlayer can be applied.

Various Thickness

A thickness of the non-magnetic support is preferably 3.0 to 6.0 μm.

A thickness of the magnetic layer is preferably equal to or smaller than0.15 μm and more preferably equal to or smaller than 0.1 μm, from aviewpoint of realization of high-density recording required in recentyears. The thickness of the magnetic layer is even more preferably 0.01to 0.1 μm. The magnetic layer may be at least single layer, the magneticlayer may be separated into two or more layers having different magneticproperties, and a configuration of a well-known multilayered magneticlayer can be applied. A thickness of the magnetic layer in a case wherethe magnetic layer is separated into two or more layers is a totalthickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.1 to 1.5 μm andis preferably 0.1 to 1.0 μm.

Meanwhile, the magnetic tape is normally used to be accommodated andcirculated in a magnetic tape cartridge. In order to increase recordingcapacity for 1 reel of the magnetic tape cartridge, it is desired toincrease a total length of the magnetic tape accommodated in 1 reel ofthe magnetic tape cartridge. In order to increase the recordingcapacity, it is necessary that the magnetic tape is thinned(hereinafter, referred to as “thinning”). As one method of thinning themagnetic tape, a method of decreasing a total thickness of a magneticlayer and a non-magnetic layer of a magnetic tape including thenon-magnetic layer and the magnetic layer on a non-magnetic support inthis order is used. In a case where the magnetic tape includes anon-magnetic layer, the total thickness of the magnetic layer and thenon-magnetic layer is preferably equal to or smaller than 1.8 μm, morepreferably equal to or smaller than 1.5 μm, and even more preferablyequal to or smaller than 1.1 μm, from a viewpoint of thinning themagnetic tape. According to the studies of the inventors, it is foundthat, in a case where the total thickness of the magnetic layer and thenon-magnetic layer is equal to or smaller than 1.1 μm and any measuresare not prepared, a decrease in resistance value of the TMR head tendsto occur even more significantly. However, even a significant decreasein resistance value of the TMR head occurring in a case where the totalthickness of the magnetic layer and the non-magnetic layer is equal toor smaller than 1.1 μm can be prevented, in a case of the magnetic tapedevice according to one aspect of the invention. In addition, the totalthickness of the magnetic layer and the non-magnetic layer can be, forexample, equal to or greater than 0.1 μm or equal to or greater than 0.2μm.

A thickness of the back coating layer is preferably equal to or smallerthan 0.9 μm and even more preferably 0.1 to 0.7 μm.

The thicknesses of various layers of the magnetic tape and thenon-magnetic support can be acquired by a well-known film thicknessmeasurement method. As an example, a cross section of the magnetic tapein a thickness direction is, for example, exposed by a well-known methodof ion beams or microtome, and the exposed cross section is observedwith a scan electron microscope. In the cross section observation,various thicknesses can be acquired as a thickness acquired at oneposition of the cross section in the thickness direction, or anarithmetical mean of thicknesses acquired at a plurality of positions oftwo or more positions, for example, two positions which are arbitrarilyextracted. In addition, the thickness of each layer may be acquired as adesigned thickness calculated according to the manufacturing conditions.

Manufacturing Method

Preparation of Each Layer Forming Composition

Each composition for forming the magnetic layer, the non-magnetic layer,or the back coating layer normally includes a solvent, together withvarious components described above. As the solvent, various organicsolvents generally used for manufacturing a coating type magneticrecording medium can be used. Among those, from a viewpoint ofsolubility of the binding agent normally used in the coating typemagnetic recording medium, each layer forming composition preferablyincludes one or more ketone solvents such as acetone, methyl ethylketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone,isophorone, and tetrahydrofuran. The amount of the solvent of each layerforming composition is not particularly limited, and can be set to bethe same as that of each layer forming composition of a typical coatingtype magnetic recording medium. In addition, steps of preparing eachlayer forming composition generally include at least a kneading step, adispersing step, and a mixing step provided before and after thesesteps, if necessary. Each step may be divided into two or more stages.All of raw materials used in the invention may be added at an initialstage or in a middle stage of each step. In addition, each raw materialmay be separately added in two or more steps. For example, in oneaspect, regarding the magnetic layer forming composition, a dispersionliquid (magnetic solution) including ferromagnetic powder and adispersion liquid (abrasive liquid) including an abrasive are separatelydispersed and prepared, and these dispersion liquids are mixed withother components at the same time or in order to prepare a magneticlayer forming composition. Other descriptions for the preparation ofeach layer forming composition, a description disclosed in a paragraph0065 of JP2010-231843A can also be referred to.

As described above, in one aspect, regarding the control of the basefriction, a magnetic tape can be manufactured by using two or more kindsof ferromagnetic powders having different average particle sizes. Thatis, the magnetic layer can be formed with first ferromagnetic powder,and one or more kinds of ferromagnetic powder having an average particlesize greater than that of the first ferromagnetic powder, asferromagnetic powder. As preferred aspects of a forming method of such amagnetic layer, aspects of the following (1) to (3) can be used. Acombination of two or more aspects described below is a more preferredaspect of the forming method of a magnetic layer. The firstferromagnetic powder is one of ferromagnetic powder among the two ormore kinds of ferromagnetic powders and is preferably ferromagneticpowder used with the largest proportion described above.

(1) An average particle size of the first ferromagnetic powder is 10 to80 nm.

(2) A difference between an average particle size of the ferromagneticpowder having an average particle size greater than that of the firstferromagnetic powder, and the average particle size of the firstferromagnetic powder is 10 to 50 nm.

(3) A mixing ratio of the first ferromagnetic powder to theferromagnetic powder having an average particle size greater than thatof the first ferromagnetic powder is 90.0:10.0 (former:latter) to99.9:0.1 based on mass.

In another aspect, a magnetic tape can also be manufactured by usingnon-magnetic powder other than the abrasive and the projection formationagent, as the non-magnetic powder of the magnetic layer. That is, themagnetic layer can be formed with the other non-magnetic powder. Aspreferred aspects of a forming method of such a magnetic layer, aspectsof the following (4) to (6) can be used. A combination of two or moreaspects described below is a more preferred aspect of the forming methodof a magnetic layer.

(4) An average particle size of the other non-magnetic powder is greaterthan an average particle size of the ferromagnetic powder.

(5) A difference between the average particle size of the ferromagneticpowder and the average particle size of the other non-magnetic powder is10 to 80 nm.

(6) A mixing ratio of the ferromagnetic powder to the other non-magneticpowder is 90.0:10.0 (former:latter) to 99.9:0.1 based on mass.

Coating Step

The magnetic layer can be formed by directly applying the magnetic layerforming composition onto the surface of the non-magnetic support orperforming multilayer coating of the magnetic layer forming compositionwith the non-magnetic layer forming composition in order or at the sametime. The back coating layer can be formed by applying the back coatinglayer forming composition to the surface of the non-magnetic supportopposite to a side provided with the magnetic layer (or to be providedwith the magnetic layer). For details of the coating for forming eachlayer, a description disclosed in a paragraph 0066 of JP2010-231843A canbe referred to.

Other Steps

For details of various other steps for manufacturing the magnetic tape,descriptions disclosed in paragraphs 0067 to 0070 of JP2010-231843A canbe referred to.

Formation of Servo Pattern

A servo pattern is formed in the magnetic layer by magnetizing aspecific position of the magnetic layer with a servo pattern recordinghead (also referred to as a “servo write head”). A well-known technologyregarding a servo pattern of the magnetic layer of the magnetic tapewhich is well known can be applied for the shapes of the servo patternwith which the head tracking servo can be performed and the dispositionthereof in the magnetic layer. For example, as a head tracking servosystem, a timing-based servo system and an amplitude-based servo systemare known. The servo pattern of the magnetic layer of the magnetic tapemay be a servo pattern capable of allowing head tracking servo of anysystem. In addition, a servo pattern capable of allowing head trackingservo in the timing-based servo system and a servo pattern capable ofallowing head tracking servo in the amplitude-based servo system may beformed in the magnetic layer.

The magnetic tape described above is generally accommodated in amagnetic tape cartridge and the magnetic tape cartridge is mounted inthe magnetic tape device. In the magnetic tape cartridge, the magnetictape is generally accommodated in a cartridge main body in a state ofbeing wound around a reel. The reel is rotatably provided in thecartridge main body. As the magnetic tape cartridge, a single reel typemagnetic tape cartridge including one reel in a cartridge main body anda twin reel type magnetic tape cartridge including two reels in acartridge main body are widely used. In a case where the single reeltype magnetic tape cartridge is mounted in the magnetic tape device(drive) in order to record and/or reproduce data (magnetic signals) tothe magnetic tape, the magnetic tape is drawn from the magnetic tapecartridge and wound around the reel on the drive side. A servo head isdisposed on a magnetic tape transportation path from the magnetic tapecartridge to a winding reel. Sending and winding of the magnetic tapeare performed between a reel (supply reel) on the magnetic tapecartridge side and a reel (winding reel) on the drive side. In themeantime, the servo head comes into contact with and slides on thesurface of the magnetic layer of the magnetic tape, and accordingly, thereading of the servo pattern is performed by the servo head. Withrespect to this, in the twin reel type magnetic tape cartridge, bothreels of the supply reel and the winding reel are provided in themagnetic tape cartridge. The magnetic tape according to one aspect ofthe invention may be accommodated in any of single reel type magnetictape cartridge and twin reel type magnetic tape cartridge. Theconfiguration of the magnetic tape cartridge is well known.

Servo Head

The magnetic tape device includes the TMR head as the servo head. TheTMR head is a magnetic head including a tunnel magnetoresistance effecttype element (TMR element). The TMR element can play a role of detectinga change in leakage magnetic field from the magnetic tape as a change inresistance value (electric resistance) by using a tunnelmagnetoresistance effect, as a servo pattern reading element for readinga servo pattern formed in the magnetic layer of the magnetic tape. Byconverting the detected change in resistance value into a change involtage, the servo pattern can be read (servo signal can be reproduced).

As the TMR head included in the magnetic tape device, a TMR head havinga well-known configuration including a tunnel magnetoresistance effecttype element (TMR element) can be used. For example, for details of thestructure of the TMR head, materials of each unit configuring the TMRhead, and the like, well-known technologies regarding the TMR head canbe used.

The TMR head is a so-called thin film head. The TMR element included inthe TMR head at least includes two electrode layers, a tunnel barrierlayer, a free layer, and a fixed layer. The TMR head includes a TMRelement in a state where cross sections of these layers face a side of asurface sliding on the magnetic tape. The tunnel barrier layer ispositioned between the two electrode layers and the tunnel barrier layeris an insulating layer. Meanwhile, the free layer and the fixed layerare magnetic layers. The free layer is also referred to as amagnetization free layer and is a layer in which a magnetizationdirection changes depending on the external magnetic field. On the otherhand, the fixed layer is a layer in which a magnetization direction doesnot change depending on the external magnetic field. The tunnel barrierlayer (insulating layer) is positioned between the two electrodes,normally, and thus, even in a case where a voltage is applied, ingeneral, a current does not flow or does not substantially flow.However, a current (tunnel current) flows by the tunnel effect dependingon a magnetization direction of the free layer affected by a leakagemagnetic field from the magnetic tape. The amount of a tunnel currentflow changes depending on a relative angle of a magnetization directionof the fixed layer and a magnetization direction of the free layer, andas the relative angle decreases, the amount of the tunnel current flowincreases. A change in amount of the tunnel current flow is detected asa change in resistance value by the tunnel magnetoresistance effect. Byconverting the change in resistance value into a change in voltage, theservo pattern can be read. For an example of the configuration of theTMR head, a description disclosed in FIG. 1 of JP2004-185676A can bereferred to, for example. However, there is no limitation to the aspectshown in the drawing. FIG. 1 of JP2004-185676A shows two electrodelayers and two shield layers. Here, a TMR head having a configuration inwhich the shield layer serves as an electrode layer is also well knownand the TMR head having such a configuration can also be used. In theTMR head, a current (tunnel current) flows between the two electrodesand thereby changing electric resistance, by the tunnelmagnetoresistance effect. The TMR head is a magnetic head having a CPPstructure, and thus, a direction in which a current flows is atransportation direction of the magnetic tape. In the invention and thespecification, the description regarding “orthogonal” includes a rangeof errors allowed in the technical field of the invention. For example,the range of errors means a range of less than ±10° from an exactorthogonal state, and the error from the exact orthogonal state ispreferably within ±5° and more preferably within ±3°. A decrease inresistance value of the TMR head means a decrease in electric resistancemeasured by bringing an electric resistance measuring device intocontact with a wiring connecting two electrodes, and a decrease inelectric resistance between two electrodes in a state where a currentdoes not flow. A significant decrease in electric resistance causes adecrease in accuracy of the head position controlling of the headtracking servo. This decrease in resistance value of the TMR head can beprevented by using a magnetic tape having the base friction equal to orsmaller than 0.30 as the magnetic tape in which the magnetic layerincludes a servo pattern.

The servo head is a magnetic head including at least the TMR element asa servo pattern reading element. The servo head may include or may notinclude a reproducing element for reproducing information recorded onthe magnetic tape. That is, the servo head and the reproducing head maybe one magnetic head or separated magnetic heads. The same applies to arecording element for performing the recording of information in themagnetic tape.

Magnetic Tape Transportation Speed

The magnetic tape transportation speed of the magnetic tape device isequal to or lower than 18 m/sec. Normally, the magnetic tapetransportation speed is set in a control unit of the magnetic tapedevice. It is desired that the magnetic tape is transported at a lowspeed in the magnetic tape device, in order to prevent a deteriorationof recording and reproducing characteristics. But, in a case where themagnetic tape transportation speed is equal to or lower than 18 m/sec inthe magnetic tape device including the TMR head as a servo head, adecrease in resistance value of the TMR head used as the servo headoccurs particularly significantly. In the magnetic tape device accordingto one aspect of the invention, such a decrease in resistance value canbe prevented by using a magnetic tape having the base friction equal toor smaller than 0.30. The magnetic tape transportation speed is equal toor lower than 18 m/sec or may be equal to or lower than 15 m/sec orequal to or lower than 10 m/sec. The magnetic tape transportation speedcan be, for example, equal to or higher than 1 m/sec.

Head Tracking Servo Method

One aspect of the invention relates to a head tracking servo methodincluding: reading a servo pattern of a magnetic layer of a magnetictape by a servo head in a magnetic tape device, a magnetic tapetransportation speed during the reproducing is equal to or lower than 18m/sec, the servo head is a magnetic head including a tunnelmagnetoresistance effect type element as a servo pattern readingelement, the magnetic tape includes a non-magnetic support, and amagnetic layer including ferromagnetic powder and a binding agent on thenon-magnetic support, the magnetic tape includes a servo pattern, and acoefficient of friction measured regarding a base portion of a surfaceof the magnetic layer is equal to or smaller than 0.30. The reading ofthe servo pattern is performed by bringing the magnetic tape intocontact with the servo head allowing sliding while transporting (causingrunning of) the magnetic tape. The details of the magnetic tape and theservo head used in the head tracking servo method are as thedescriptions regarding the magnetic tape device according to one aspectof the invention.

Hereinafter, as one specific aspect of the head tracking servo, headtracking servo in the timing-based servo system will be described.However, the head tracking servo of the invention is not limited to thefollowing specific aspect.

In the head tracking servo in the timing-based servo system(hereinafter, referred to as a “timing-based servo”), a plurality ofservo patterns having two or more different shapes are formed in amagnetic layer, and a position of a servo head is recognized by aninterval of time in a case where the servo head has read the two servopatterns having different shapes and an interval of time in a case wherethe servo head has read two servo patterns having the same shapes. Theposition of the magnetic head of the magnetic tape in the widthdirection is controlled based on the position of the servo headrecognized as described above. In one aspect, the magnetic head, theposition of which is controlled here, is a magnetic head (reproducinghead) which reproduces information recorded on the magnetic tape, and inanother aspect, the magnetic head is a magnetic head (recording head)which records information in the magnetic tape.

FIG. 1 shows an example of disposition of data bands and servo bands. InFIG. 1, a plurality of servo bands 10 are disposed to be interposedbetween guide bands 12 in a magnetic layer of a magnetic tape 1. Aplurality of regions 11 each of which is interposed between two servobands are data bands. The servo pattern is a magnetized region and isformed by magnetizing a specific region of the magnetic layer by a servowrite head. The region magnetized by the servo write head (positionwhere a servo pattern is formed) is determined by standards. Forexample, in an LTO Ultrium format tape which is based on a localstandard, a plurality of servo patterns tilted in a tape width directionas shown in FIG. 2 are formed on a servo band in a case of manufacturinga magnetic tape. Specifically, in FIG. 2, a servo frame SF on the servoband 10 is configured with a servo sub-frame 1 (SSF1) and a servosub-frame 2 (SSF2). The servo sub-frame 1 is configured with an A burst(in FIG. 2, reference numeral A) and a B burst (in FIG. 2, referencenumeral B). The A burst is configured with servo patterns A1 to A5 andthe B burst is configured with servo patterns B1 to B5. Meanwhile, theservo sub-frame 2 is configured with a C burst (in FIG. 2, referencenumeral C) and a D burst (in FIG. 2, reference numeral D). The C burstis configured with servo patterns C1 to C4 and the D burst is configuredwith servo patterns D1 to D4. Such 18 servo patterns are disposed in thesub-frames in the arrangement of 5, 5, 4, 4, as the sets of 5 servopatterns and 4 servo patterns, and are used for recognizing the servoframes. FIG. 2 shows one servo frame for explaining. However, inpractice, in the magnetic layer of the magnetic tape in which the headtracking servo in the timing-based servo system is performed, aplurality of servo frames are disposed in each servo band in a runningdirection. In FIG. 2, an arrow shows the running direction. For example,an LTO Ultrium format tape generally includes 5,000 or more servo framesper a tape length of 1 m, in each servo band of the magnetic layer. Theservo head sequentially reads the servo patterns in the plurality ofservo frames, while coming into contact with and sliding on the surfaceof the magnetic layer of the magnetic tape transported in the magnetictape device.

In the head tracking servo in the timing-based servo system, a positionof a servo head is recognized based on an interval of time in a casewhere the servo head has read the two servo patterns (reproduced servosignals) having different shapes and an interval of time in a case wherethe servo head has read two servo patterns having the same shapes. Thetime interval is normally obtained as a time interval of a peak of areproduced waveform of a servo signal. For example, in the aspect shownin FIG. 2, the servo pattern of the A burst and the servo pattern of theC burst are servo patterns having the same shapes, and the servo patternof the B burst and the servo pattern of the D burst are servo patternshaving the same shapes. The servo pattern of the A burst and the servopattern of the C burst are servo patterns having the shapes differentfrom the shapes of the servo pattern of the B burst and the servopattern of the D burst. An interval of the time in a case where the twoservo patterns having different shapes are read by the servo head is,for example, an interval between the time in a case where any servopattern of the A burst is read and the time in a case where any servopattern of the B burst is read. An interval of the time in a case wherethe two servo patterns having the same shapes are read by the servo headis, for example, an interval between the time in a case where any servopattern of the A burst is read and the time in a case where any servopattern of the C burst is read. The head tracking servo in thetiming-based servo system is a system supposing that occurrence of adeviation of the time interval is due to a position change of themagnetic tape in the width direction, in a case where the time intervalis deviated from the set value. The set value is a time interval in acase where the magnetic tape runs without occurring the position changein the width direction. In the timing-based servo system, the magnetichead is moved in the width direction in accordance with a degree of thedeviation of the obtained time interval from the set value.Specifically, as the time interval is greatly deviated from the setvalue, the magnetic head is greatly moved in the width direction. Thispoint is applied to not only the aspect shown in FIGS. 1 and 2, but alsoto entire timing-based servo systems.

For the details of the head tracking servo in the timing-based servosystem, well-known technologies such as technologies disclosed in U.S.Pat. No. 5,689,384A, U.S. Pat. No. 6,542,325B, and U.S. Pat. No.7,876,521B can be referred to, for example. In addition, for the detailsof the head tracking servo in the amplitude-based servo system,well-known technologies disclosed in U.S. Pat. No. 5,426,543A and U.S.Pat. No. 5,898,533A can be referred to, for example.

According to one aspect of the invention, a magnetic tape used in amagnetic tape device in which a TMR head is used as a servo head and amagnetic tape transportation speed in a case of reading a servo patternof a magnetic layer of the magnetic tape is equal to or lower than 18m/sec, the magnetic tape including: a magnetic layer includingferromagnetic powder and a binding agent on a non-magnetic support, inwhich the magnetic layer includes a servo pattern, and a coefficient offriction measured regarding a base portion of a surface of the magneticlayer is equal to or smaller than 0.30 is also provided. The details ofthe magnetic tape is also as the descriptions regarding the magnetictape device according to one aspect of the invention.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to aspects shown in the examples.“Parts” and “%” in the following description mean “parts by mass” and“mass %”, unless otherwise noted.

Example 1

1. Manufacturing of Magnetic Tape

(1) Preparation of Alumina Dispersion

3.0 parts of 2,3-dihydroxynaphthalene (manufactured by Tokyo ChemicalIndustry Co., Ltd.), 31.3 parts of a 32% solution (solvent is a mixedsolvent of methyl ethyl ketone and toluene) of a polyester polyurethaneresin having a SO₃Na group as a polar group (UR-4800 (amount of a polargroup: 80 meq/kg) manufactured by Toyobo Co., Ltd.), and 570.0 parts ofa mixed solution of methyl ethyl ketone and cyclohexanone (mass ratio of1:1) as a solvent were mixed in 100.0 parts of alumina powder (HIT-70manufactured by Sumitomo Chemical Co., Ltd.) having an gelatinizationratio of 65% and a BET specific surface area of 30 m²/g, and dispersedin the presence of zirconia beads by a paint shaker for 5 hours. Afterthe dispersion, the dispersion liquid and the beads were separated by amesh and an alumina dispersion was obtained.

(2) Magnetic Layer Forming Composition List

Magnetic Solution

Ferromagnetic hexagonal barium ferrite powder: 100.0 parts

-   -   Two kinds of ferromagnetic hexagonal barium ferrite powders        below are used    -   Ferromagnetic hexagonal barium ferrite powder (1)    -   Average particle size and amount used: see Table 1    -   Ferromagnetic hexagonal barium ferrite powder (2)    -   Average particle size and amount used: see Table 1

SO₃Na group-containing polyurethane resin: 14.0 parts

-   -   Weight-average molecular weight: 70,000, SO₃Na group: 0.2 meq/g

Cyclohexanone: 150.0 parts

Methyl ethyl ketone: 150.0 parts

Abrasive Liquid

Alumina dispersion prepared in the section (1): 6.0 parts

Silica Sol

Colloidal silica: 2.0 parts

-   -   Average particle size: see Table 1

Methyl ethyl ketone: 1.4 parts

Other Components

Stearic acid: 2.0 parts

Butyl stearate: 6.0 parts

Polyisocyanate (CORONATE (registered trademark) manufactured by NipponPolyurethane Industry Co., Ltd.): 2.5 parts

Finishing Additive Solvent

Cyclohexanone: 200.0 parts

Methyl ethyl ketone: 200.0 parts

(3) Non-Magnetic Layer Forming Composition List

Non-magnetic inorganic powder: α-iron oxide: 100.0 parts

-   -   Average particle size (average long axis length): 100 nm    -   Average acicular ratio: 1.9    -   BET specific surface area: 75 m²/g

Carbon black: 20.0 parts

-   -   Average particle size: 20 nm

SO₃Na group-containing polyurethane resin: 18.0 parts

-   -   Weight-average molecular weight: 70,000, SO₃Na group: 0.2 meq/g

Stearic acid: 1.0 part

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

(4) Back Coating Layer Forming Composition List

Non-magnetic inorganic powder: α-iron oxide: 80.0 parts

-   -   Average particle size (average long axis length): 0.15 μm    -   Average acicular ratio: 7    -   BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

-   -   Average particle size: 20 nm

A vinyl chloride copolymer: 13.0 parts

Sulfonic acid group-containing polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Stearic acid: 3.0 parts

Butyl stearate: 3.0 parts

Polyisocyanate: 5.0 parts

Cyclohexanone: 355.0 parts

Methyl ethyl ketone: 155.0 parts

(5) Preparation of Each Layer Forming Composition

The magnetic layer forming composition was prepared by the followingmethod. The magnetic solution was prepared by dispersing(beads-dispersing) each component with a batch type vertical sand millfor 24 hours. As the dispersion beads, zirconia beads having a beaddiameter of 0.5 mm were used. The prepared magnetic solution and theabrasive liquid were mixed with other components (silica sol, othercomponents, and finishing additive solvent) and beads-dispersed for 5minutes by using the sand mill, and a process (ultrasonic dispersion)was performed with a batch type ultrasonic device (20 kHz, 300 W) for0.5 minutes. After that, the filtering was performed by using a filterhaving an average hole diameter of 0.5 μm, and the magnetic layerforming composition was prepared.

The non-magnetic layer forming composition was prepared by the followingmethod. Each component excluding stearic acid, cyclohexanone, and methylethyl ketone was dispersed by using batch type vertical sand mill for 24hours to obtain a dispersion liquid. As the dispersion beads, zirconiabeads having a bead diameter of 0.1 mm were used. After that, theremaining components were added into the obtained dispersion liquid andstirred with a dissolver. The dispersion liquid obtained as describedabove was filtered with a filter having an average hole diameter of 0.5μm and a non-magnetic layer forming composition was prepared.

The back coating layer forming composition was prepared by the followingmethod. Each component excluding the lubricant (stearic acid and butylstearate), polyisocyanate, and cyclohexanone was kneaded and diluted byan open kneader, and subjected to a dispersion process of 12 passes,with a transverse beads mill dispersing device and zirconia beads havinga bead diameter of 1 mm, by setting a bead filling percentage as 80volume %, a circumferential speed of rotor distal end as 10 m/sec, and aretention time for 1 pass as 2 minutes. After that, the remainingcomponents were added into the obtained dispersion liquid and stirredwith a dissolver. The dispersion liquid obtained as described above wasfiltered with a filter having an average hole diameter of 1 μm and aback coating layer forming composition was prepared.

(6) Manufacturing Method of Magnetic Tape

The non-magnetic layer forming composition prepared in the section (5)was applied to a surface of a support made of polyethylene naphthalatehaving a thickness of 5.0 μm so that the thickness after the dryingbecomes a thickness shown in Table 1, and then, the magnetic layerforming composition prepared in the section (5) was applied thereon sothat the thickness after the drying becomes a thickness shown in Table1, and a coating layer was formed. A homeotropic alignment process wasperformed by applying a magnetic field having a magnetic field strengthof 0.3 T to the surface of the coating layer of the magnetic layerforming composition in a vertical direction while the coating layer isnot dried, and then, the coating layer was dried. After that, the backcoating layer forming composition prepared in the section (5) wasapplied to the surface of the non-magnetic support made of polyethylenenaphthalate on a side opposite to the surface where the non-magneticlayer and the magnetic layer are formed, so that the thickness after thedrying becomes 0.5 μm, and then drying was performed.

After that, a calender process (surface smoothing treatment) wasperformed with a calender roll configured of only a metal roll, at aspeed of 100 m/min, linear pressure of 294 kN/m (300 kg/cm), and acalender temperature (surface temperature of a calender roll) shown inTable 1.

Then, a heating process was performed in the environment of theatmosphere temperature of 70° C. for 36 hours. After the heatingprocess, the layer was slit to have a width of ½ inches (0.0127 meters),and a magnetic tape for forming a servo pattern on the magnetic layerwas manufactured.

In a state where the magnetic layer of the manufactured magnetic tapewas demagnetized, servo patterns having disposition and shapes accordingto the LTO Ultrium format were formed on the magnetic layer by using aservo write head mounted on a servo tester. Accordingly, a magnetic tapeincluding data bands, servo bands, and guide bands in the dispositionaccording to the LTO Ultrium format in the magnetic layer, and includingservo patterns having the disposition and the shape according to the LTOUltrium format on the servo band is manufactured. The servo testerincludes a servo write head and a servo head. This servo tester was alsoused in evaluations which will be described later.

The thickness of each layer of the manufactured magnetic tape wasacquired by the following method. It was confirmed that the thicknessesof the formed non-magnetic layer and the magnetic layer were thethicknesses shown in Table 1 and the thicknesses of the back coatinglayer and the non-magnetic support were the thicknesses described above.

A cross section of the magnetic tape in a thickness direction wasexposed to ion beams and the exposed cross section was observed with ascanning electron microscope. Various thicknesses were obtained as anarithmetical mean of thicknesses obtained at two portions in thethickness direction in the cross section observation.

A part of the magnetic tape manufactured by the method described abovewas used in the evaluation described below, and the other part was usedin order to measure a resistance value of the TMR head which will bedescribed later.

The amount of the ferromagnetic hexagonal barium ferrite powder shown inTable 1 is content of each ferromagnetic hexagonal barium ferrite powderbased on mass with respect to 100.0 parts by mass of a total of theferromagnetic hexagonal barium ferrite powder. An average particle sizeof the ferromagnetic hexagonal barium ferrite powder shown in Table 1 isa value obtained by collecting the necessary amount from a batch of thepowder used in the preparation of the magnetic tape and measuring anaverage particle size by the method described above. The ferromagnetichexagonal barium ferrite powder after measuring the average particlesize was used in the preparation of a magnetic solution for preparingthe magnetic tape.

2. Evaluation of Physical Properties of Magnetic Tape

(1) Center Line Average Surface Roughness Ra Measured Regarding Surfaceof Magnetic Layer

The measurement regarding a measurement area of 40 μm×40 μm in thesurface of the magnetic layer of the magnetic tape was performed with anatomic force microscope (AFM, Nanoscope 4 manufactured by VeecoInstruments, Inc.), and a center line average surface roughness Ra wasacquired. A scan speed (probe movement speed) was set as 40 μm/sec and aresolution was set as 512 pixel×512 pixel.

(2) Base Friction

First, marking was performed on a measurement surface with a lasermarker in advance, and an atomic force microscope (AFM) image of aportion separated from the mark by a certain distance (approximately 100μm) was observed. The observation was performed regarding an area of avisual field of 7 μm×7 μm. At this time, marking was performed on theARM by changing a cantilever to a hard material (single crystalsilicon), so as to easily capture a scanning electron microscope (SEM)image of the same portion as will be described later. All of projectionshaving a height equal to or greater than 15 nm from the referencesurface were extracted from the AFM image observed as described above. Aportion in which it is determined that projections were not present, wasspecified as a base portion, and the base friction was measured withTI-950 type Tribolndenter manufactured by Hysitron, Inc. by the methoddescribed above.

An SEM image of the same portion as the portion observed with the AFMwas observed to obtain a component map, and it was confirmed that theextracted projections having a height equal to or greater than 15 nmfrom the reference surface were projections formed of alumina orcolloidal silica. In Examples 1 to 8, in the component map obtained withthe SEM, alumina and colloidal silica were not confirmed in the baseportion. Here, the component analysis was performed with the SEM, butthe component analysis is not limited to being performed with the SEM,and can be performed by a well-known method such as energy dispersiveX-ray spectrometry (EDS) or auger electron spectroscopy (AES).

3. Measurement of Resistance Value of Servo Head

The servo head of the servo tester was replaced with a commerciallyavailable TMR head (element width of 70 nm) as a reproducing head forHDD. In the servo tester, the magnetic tape manufactured in the part 1.was transported while bringing the surface of the magnetic layer intocontact with the servo head and causing sliding therebetween. A tapelength of the magnetic tape was 1,000 m, and a total of 4,000 passes ofthe transportation (running) of the magnetic tape was performed bysetting the magnetic tape transportation speed (relative speed of themagnetic tape and the servo head) at the time of the transportation as avalue shown in Table 1. The servo head was moved in a width direction ofthe magnetic tape by 2.5 μm for 1 pass, a resistance value (electricresistance) of the servo head for transportation of 400 passes wasmeasured, and a rate of a decrease in resistance value with respect toan initial value (resistance value at 0 pass) was obtained by thefollowing equation.Rate of decrease in resistance value (%)=[(initial value−resistancevalue after transportation of 400 passes)/initial value]×100

The measurement of the resistance value (electric resistance) wasperformed by bringing an electric resistance measuring device (digitalmulti-meter (product number: DA-50C) manufactured by Sanwa ElectricInstrument Co., Ltd.) into contact with a wiring connecting twoelectrodes of a TMR element included in a TMR head. In a case where thecalculated rate of a decrease in resistance value was equal to orgreater than 30%, it was determined that a decrease in resistance valueoccurred. Then, a servo head was replaced with a new head, andtransportation after 400 passes was performed and a resistance value wasmeasured. The number of times of occurrence of a decrease in resistancevalue which is 1 or greater indicates a significant decrease inresistance value. In the running of 4,000 passes, in a case where therate of a decrease in resistance value did not become equal to orgreater than 30%, the number of times of occurrence of a decrease inresistance value was set as 0. In a case where the number of times ofoccurrence of a decrease in resistance value is 0, the maximum value ofthe measured rate of a decrease in resistance value is shown in Table 1.

Examples 2 to 8 and Comparative Examples 1 to 14

1. Manufacturing of Magnetic Tape

A magnetic tape was manufactured in the same manner as in Example 1,except that various conditions shown in Table 1 were changed as shown inTable 1.

2. Evaluation of Physical Properties of Magnetic Tape

Various physical properties of the manufactured magnetic tape wereevaluated in the same manner as in Example 1.

3. Measurement of Resistance Value of Servo Head

A resistance value of the servo head was measured by the same method asthat in Example 1, by using the manufactured magnetic tape. The magnetictape transportation speed was set as a value shown in Table 1. InExamples 2 to 8 and Comparative Examples 7 to 14, the TMR head which wasthe same as that in Example 1 was used as a servo head. In ComparativeExamples 1 to 6, a commercially available spin valve type GMR head(element width of 70 nm) was used as a servo head. This GMR head was amagnetic head having a OP structure including two electrodes with an MRelement interposed therebetween in a direction orthogonal to thetransportation direction of the magnetic tape. A resistance value wasmeasured in the same manner as in Example 1, by bringing an electricresistance measuring device into contact with a wiring connecting thesetwo electrodes.

The results of the evaluations described above are shown in Table 1.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Example 7 Example 8 Magnetic layer thickness 0.1 μm 0.1 μm 0.1 μm 0.1 μm0.1 μm 0.1 μm 0.1 μm 0.1 μm Non-magnetic layer thickness 1.5 μm 1 μm 1μm 0.5 μm 0.5 μm 0.3 μm 0.3 μm 0.3 μm Total thickness of magneticlayer + 1.6 μm 1.1 μm 1.1 μm 0.6 μm 0.6 μm 0.4 μm 0.4 μm 0.4 μmnon-magnetic layer Colloidal silica average particle size 120 nm 80 nm80 nm 80 nm 80 nm 40 nm 40 nm 40 nm Calender temperature 80° C. 90° C.90° C. 90° C. 90° C. 110° C. 110° C. 110° C. Center line average surfaceroughness Ra 2.8 nm 2.0 nm 2.0 nm 2.0 nm 2.0 nm 1.5 nm 1.5 nm 1.5 nmFerromagnetic hexagonal Average particle 25 nm 25 nm 25 nm 25 nm 25 nm25 nm 25 nm 25 nm barium ferrite powder (1) size Amount used 99.0% 99.0%98.7% 98.7% 98.5% 98.5% 98.5% 98.5% Ferromagnetic hexagonal Averageparticle 55 nm 55 nm 55 nm 55 nm 55 nm 55 nm 55 nm 55 nm barium ferritepowder (2) size Amount used 1.0% 1.0% 1.3% 1.3% 1.5% 1.5% 1.5% 1.5% Basefriction 0.28 0.28 0.26 0.26 0.23 0.23 0.23 0.23 Servo head TMR TMR TMRTMR TMR TMR TMR TMR Magnetic tape transportation speed 18 m/sec 18 m/sec18 m/sec 18 m/sec 18 m/sec 18 m/sec 10 m/sec 1 m/sec Number of times ofoccurrence of decrease 0 0 0 0 0 0 0 0 in resistance value (times) Rateof decrease in resistance value (%) 8 9 6 15 5 6 13 18 ComparativeComparative Comparative Comparative Comparative Comparative ComparativeExample 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7Magnetic layer thickness 0.1 μm 0.1 μm 0.1 μm 0.1 μm 0.1 μm 0.1 μm 0.1μm Non-magnetic layer thickness 1.5 μm 1.0 μm 1.0 μm 0.5 μm 0.5 μm 0.5μm 1.5 μm Total thickness of magnetic layer + non-magnetic 1.6 μm 1.1 μm1.1 μm 0.6 μm 0.6 μm 0.6 μm 1.6 μm layer Colloidal silica averageparticle size 120 nm 120 nm 80 nm 80 nm 80 nm 80 nm 120 nm Calendertemperature 80° C. 90° C. 90° C. 80° C. 90° C. 90° C. 80° C. Center lineaverage surface roughness Ra 2.8 nm 2.5 nm 2.0 nm 2.5 nm 2.0 nm 2.0 nm2.8 nm Ferromagnetic hexagonal Average particle size 25 nm 25 nm 25 nm25 nm 25 nm 25 nm 25 nm barium ferrite powder (1) Amount used 100.0%100.0% 100.0% 100.0% 100.0% 100.0% 100.0% Ferromagnetic hexagonalAverage particle size — — — — — — — barium ferrite powder (2) Amountused — — — — — — — Base friction 0.45 0.45 0.45 0.45 0.45 0.45 0.45Servo head GMR GMR GMR GMR GMR GMR TMR Magnetic tape transportationspeed 18 m/sec 18 m/sec 18 m/sec 18 m/sec 18 m/sec 1 m/sec 19 m/secNumber of times of occurrence of decrease in 0 0 0 0 0 0 0 resistancevalue (times) Rate of decrease in resistance value (%) 0 0 0 0 0 0 0Comparative Comparative Comparative Comparative Comparative ComparativeComparative Example 8 Example 9 Example 10 Example 11 Example 12 Example13 Example 14 Magnetic layer thickness 0.1 μm 0.1 μm 0.1 μm 0.1 μm 0.1μm 0.1 μm 0.1 μm Non-magnetic layer thickness 1.5 μm 1.0 μm 1.0 μm 0.5μm 0.5 μm 0.3 μm 1.0 μm Total thickness of magnetic layer + non-magnetic1.6 μm 1.1 μm 1.1 μm 0.6 μm 0.6 μm 0.4 μm 1.1 μm layer Colloidal silicaaverage particle size 120 nm 120 nm 80 nm 80 nm 80 nm 40 nm 80 nmCalender temperature 80° C. 90° C. 90° C. 80° C. 90° C. 110° C. 90° C.Center line average surface roughness Ra 2.8 nm 2.5 nm 2.0 nm 2.5 nm 2.0nm 1.5 nm 2.0 nm Ferromagnetic hexagonal Average particle size 25 nm 25nm 25 nm 25 nm 25 nm 25 nm 25 nm barium ferrite powder (1) Amount used100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 99.2% Ferromagnetic hexagonalAverage particle size — — — — — — 55 nm barium ferrite powder (2) Amountused — — — — — — 0.8% Base friction 0.45 0.45 0.45 0.45 0.45 0.45 0.35Servo head TMR TMR TMR TMR TMR TMR TMR Magnetic tape transportationspeed 18 m/sec 18 m/sec 18 m/sec 18 m/sec 18 m/sec 18 m/sec 18 m/secNumber of times of occurrence of decrease in 1 3 7 9 10 10 1 resistancevalue (times) Rate of decrease in resistance value (%) — — — — — — —

As shown in Table 1, in Comparative Examples 1 to 6 in which the GMRhead was used as a servo head, the magnetic tape transportation speedwas equal to or lower than 18 m/sec and, even in a case where the basefriction of the magnetic tape exceeded 0.30, a significant decrease inresistance value of the servo head was not observed. In addition, inComparative Example 7 in which the magnetic tape transportation speedexceeded 18 m/sec although the TMR head was used as a servo head, evenin a case where the base friction of the magnetic tape exceeded 0.30, asignificant decrease in resistance value of the servo head was notobserved. On the other hand, in Comparative Examples 8 to 14 in whichthe TMR head was used as a servo head, the magnetic tape transportationspeed was equal to or lower than 18 m/sec, and a case where the basefriction of the magnetic tape exceeded 0.30, a significant decrease inresistance value of the servo head occurred.

With respect to this, in Examples 1 to 8 in which the TMR head was usedas a servo head, the magnetic tape transportation speed was equal to orlower than 18 m/sec, and the base friction of the magnetic tape equal toor smaller than 0.30, it was possible to prevent a significant decreasein resistance value of the servo head (TMR head).

One aspect of the invention is effective for usage of magnetic recordingfor which high-sensitivity reproducing of information recorded with highdensity is desired.

What is claimed is:
 1. A magnetic tape device comprising: a magnetictape; and a servo head, wherein a magnetic tape transportation speed ofthe magnetic tape device is equal to or lower than 18 m/sec, the servohead is a magnetic head including a tunnel magnetoresistance effect typeelement as a servo pattern reading element, the magnetic tape includes anon-magnetic support, and a magnetic layer including ferromagneticpowder and a binding agent on the non-magnetic support, the magneticlayer includes a servo pattern, and a coefficient of friction measuredregarding a base portion of a surface of the magnetic layer is equal toor smaller than 0.30.
 2. The magnetic tape device according to claim 1,wherein the coefficient of friction measured regarding the base portionof the surface of the magnetic layer is 0.20 to 0.30.
 3. The magnetictape device according to claim 1, wherein a center line average surfaceroughness Ra measured regarding a surface of the magnetic layer is equalto or smaller than 2.8 nm.
 4. The magnetic tape device according toclaim 3, wherein the center line average surface roughness Ra is equalto or smaller than 2.5 nm.
 5. The magnetic tape device according toclaim 1, wherein the magnetic tape includes a non-magnetic layerincluding non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer, and a total thickness ofthe magnetic layer and the non-magnetic layer is equal to or smallerthan 1.8 μm.
 6. The magnetic tape device according to claim 5, whereinthe total thickness of the magnetic layer and the non-magnetic layer isequal to or smaller than 1.1 μm.
 7. A head tracking servo methodcomprising: reading a servo pattern of a magnetic layer of a magnetictape by a servo head in a magnetic tape device, wherein a magnetic tapetransportation speed of the magnetic tape device is equal to or lowerthan 18 m/sec, the servo head is a magnetic head including a tunnelmagnetoresistance effect type element as a servo pattern readingelement, the magnetic tape includes a non-magnetic support, and amagnetic layer including ferromagnetic powder and a binding agent on thenon-magnetic support, the magnetic layer includes the servo pattern, anda coefficient of friction measured regarding a base portion of a surfaceof the magnetic layer is equal to or smaller than 0.30.
 8. The headtracking servo method according to claim 7, wherein the coefficient offriction measured regarding the base portion of the surface of themagnetic layer is 0.20 to 0.30.
 9. The head tracking servo methodaccording to claim 7, wherein a center line average surface roughness Rameasured regarding a surface of the magnetic layer is equal to orsmaller than 2.8 nm.
 10. The head tracking servo method according toclaim 9, wherein the center line average surface roughness Ra is equalto or smaller than 2.5 nm.
 11. The head tracking servo method accordingto claim 7, wherein the magnetic tape includes a non-magnetic layerincluding non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer, and a total thickness ofthe magnetic layer and the non-magnetic layer is equal to or smallerthan 1.8 μm.
 12. The head tracking servo method according to claim 11,wherein the total thickness of the magnetic layer and the non-magneticlayer is equal to or smaller than 1.1 μm.